1. Field of the Invention
The present invention relates to thin films suitable as dielectrics in IC's and for other similar applications. In particular, the invention concerns thin films comprising compositions obtainable by polymerization of silicon containing monomers, which yield an at least partially cross-linked siloxane structure. The invention also concerns a method for producing such films by preparing siloxane compositions by polymerization of the monomers, by applying the polymerized compositions on a substrate in the form of a layer and by curing the layer to form a film. Further, the invention concerns integrated circuit devices and methods of manufacturing them.
2. Description of Related Art
Built on semiconductor substrates, integrated circuits comprise millions of transistors and other devices, which communicate electrically with one another and with outside packaging materials through multiple levels of vertical and horizontal wiring embedded in a dielectric material. Within the metallization structure, “vias” make up the vertical wiring, whereas “interconnects” form the horizontal wiring. Fabricating the metallization can involve the successive depositing and patterning of multiple layers of dielectric and metal to achieve electrical connection among transistors and to outside packaging material. The patterning for a given layer is often performed by a multi-step process comprising layer deposition, photoresist spin, photoresist exposure, photoresist develop, layer etch, and photoresist removal on a substrate. Alternatively, the metal may sometimes be patterned by first etching patterns into a layer of a dielectric material, filling the pattern with metal, then subsequently chemically/mechanically polishing the metal so that the metal remains embedded only in the openings of the dielectric. As an interconnect material, aluminum has been utilized for many years due to its high conductivity, good adhesion to SiO2, known processing methods (sputtering and etching) and low cost. Aluminum alloys have also been developed over the years to improve the melting point, diffusion, electromigration and other qualities as compared to pure aluminum. Spanning successive layers of aluminum, tungsten has traditionally served as the conductive via plug material.
In IC's, silicon dioxide, having a dielectric constant of around 4.0, has been the dielectric of choice, used in conjunction with aluminum-based and tungsten-based interconnects and via for many years.
The drive to faster microprocessors and more powerful electronic devices in recent years has resulted in very high circuit densities and faster operating speeds which—in turn—have required that higher conductivity metals and significantly lower-k dielectrics compared to silicon dioxide (preferably below 3.0) be used. In the past few years, VLSI (and ULSI) processes have been moving to copper damascene processes, where copper (or a copper alloy) is used for the higher conductance in the conductor lines and a spin-on or CVD process is used for producing low-k dielectrics which can be employed for the insulating material surrounding the conductor lines. To circumvent problems with etching, copper along with a barrier metal is blanket deposited over recessed dielectric structures consisting of interconnect and via openings and subsequently polished in a processing method known as the “dual damascene.” The bottom of the via opening is usually the top of an interconnect from the previous metal layer or, in some instances, the contacting layer to the substrate.
Summarizing: aside from possessing a low dielectric constant, the ideal dielectric should have the following properties:    1. High modulus and hardness in order to bind the maze of metal interconnects and vias together in particular in the final chip packaging step as well as abet chemical mechanical polishing processing steps.    2. Low thermal expansion, typically less than or equal to that of metal interconnects.    3. Excellent thermal stability, generally in excess of 400° C., but more often even better than 500° C.    4. No cracking even as thick films structures, excellent fill and planarization properties.    5. Excellent adhesion to dielectric, semiconductor, diffusion barrier and metal materials.    6. Sufficient thermal conductivity to dissipate joule heating from interconnects and vias.    7. Material density that precludes absorption of solvents, moisture, or reactive gasses.    8. Allows desired etch profiles at very small dimensions.    9. Low current leakage, high breakdown voltages, and low loss-tangents.    10. Stable interfaces between the dielectric and contacting materials.
By necessity, low-k materials are usually engineered on the basis of compromises.
Organic polymers can be divided into two different groups with respect to the behavior of their dielectric constant. Non-polar polymers contain molecules with almost purely covalent bonds. Since they mainly consist of non-polar C-C bonds, the dielectric constant can be estimated using only density and chemical composition. Polar polymers do not have low loss, but rather contain atoms of different electronegativity, which give rise to an asymmetric charge distribution. Thus polar polymers have higher dielectric loss and a dielectric constant, which depends on the frequency and temperature at which they are evaluated. Several organic polymers have been developed for dielectric purposes. However, applicability of these films is limited because of their low thermal stability, softness, and incompatibility with traditional technological processes developed for SiO2 based dielectrics.
Therefore most of the current developments are focusing on SSQ (silsesquioxane or siloxane) or silica based dielectric materials. For SSQ based materials, silsesquioxane (siloxane) is the elementary unit. Silsesquioxanes, or T-resins, are organic-inorganic hybrid polymers with the empirical formula (R—SiO3/2)n. The most common representative of these materials comprise a ladder-type structure, and a cage structure containing eight silicon atoms placed at the vertices of a cube (T8 cube) on silicon can include hydrogen, alkyl, alkenyl, alkoxy, and aryl. Many silsesquioxanes have reasonably good solubility in common organic solvents due to their organic substitution on Si. The organic substitutes provide low density and low dielectric constant matrix material. The lower dielectric constant of the matrix material is also attributed to a low polarizability of the Si—R bond in comparison with the Si—O bond in SiO2. The silsesquioxane based materials for microelectronic application are mainly hydrogen-silsesquioxane, HSQ, and methyl-silsesquioxane, (CH3—SiO3/2)n (MSQ). MSQ materials have a lower dielectric constant as compared to HSQ because of the larger size of the CH3 group ˜2.8 and 3.0-3.2, respectively and lower polarizability of the Si—CH3 bond as compared to Si—H.
The silica-based materials have the tetrahedral basic structure of SiO2. Silica has a molecular structure in which each Si atom is bonded to four oxygen atoms. Each silicon atom is at the center of a regular tetrahedron of oxygen atoms, i.e., it forms bridging crosslinks. All pure of silica have dense structures and high chemical and excellent thermal stability. For example, amorphous silica films, used in microelectronics, have a density of 2.1 to 2.2 g/cm3. However, their dielectric constant is also high ranging from 4.0 to 4.2 due to high frequency dispersion of the dielectric constant which is related to the high polarizability of the Si—O bonds. Therefore, it is necessary to replace one or more Si—O—Si bridging groups with C-containing organic groups, such as CH3 groups, which lowers the k-value. However, these organic units reduce the degrees of bridging crosslinks as well increases the free volume between the molecules due to steric hindrance. Therefore, their mechanic strength (Young's modulus <6 GPa) and chemical resistance is reduced compared to tetrahedral silicon dioxide. Also, these methyl-based silicate and SSQ (i.e., MSQ) polymers have relatively low cracking threshold, typically on the order of 1 um or less.
Quite recently there have been some efforts to develop enhanced MSQ polymers by co-polymerizing them with disilanes, i.e., bistrimethoxysilane, that contain bridging alkyl groups between silanes and thus crosslinking density has been increased. However, these materials still contain significant amount of methyl-based silanes, i.e. methyl-trimethoxysilane, as comonomers and due to methyl co-polymer nature only moderate Young's modulus and hardness properties has been obtained, with dielectric constant of around 2.93.